Introduction

This paper presents the three main plant
diseases in south-west Australia - Phytophthora,
Armillaria, and cankers - and discusses the threat these fungi
pose for conservation values in the State.

Organisms that cause plant diseases are called pathogens. They include microorganisms such as fungi, bacteria and viruses, some species of protozoas and nematodes. Pathogenic organisms are usually a normal component of the soil population and naturally exist in relatively low numbers. Some pathogenic species only cause disease in one species of plant, but others can cause disease on plants that are not closely related to each other.

To enter a plant root the bacteria or fungi must first be present in the rhizosphere of the plant. It then uses molecular signals to recognise whether the plant root is susceptible to entry or not. The pathogen attaches to the root surface possibly by the use of hair-like structures and enters the root. Some pathogens enter through areas that have been damaged by animals and some fungi and bacteria produce enzymes that dissolve the chemical compounds that make up the cell wall. Once the pathogen is in the plant cells, the plant may try to prevent its spread by producing chemical or physical barriers. These procedures may confine the pathogen to a portion of the root.

Dieback Disease

The disease known variously as "jarrah dieback", "wildflower
dieback", or "dieback disease" has long been recognised as a serious
threat to the conservation of both flora and fauna in Western
Australia, especially in the infertile Northern and Southern
Sandplain regions of the state where the greatest diversity of plants
are found. The disease is caused by a soil-borne pathogen (technically classified as a water mould, not a fungus), Phytophthora cinnamomi. The unwitting movement of
pathogen-infested mud by vehicles travelling from infested into healthy
areas is by far the most important means of dispersal of the organism,
although movement of the pathogen in mud carried along trails by both
bushwalkers and animals can also be important.

Impact of dieback on a stand of jarrah at the Ronson Formation
near Dwellingup, Western Australia.

The scientific name "Phytophthora" (pronounced "fy-tof-thor-a" -
derived from Greek "phyton" meaning "plant" and "phthora" meaning
"destruction"). Many plant species are killed by this water mould, and
important families of plants from Western Australia contain a large
number of susceptible species. In 1980, Phytophthora cinnamomi
had been isolated from 967 plant species world-wide, with nearly
half of these records from Australia. Other species of
Phytophthora also pose a threat and are also capable of
killing a large range of native plants. Although it is not known how
many species of plants might be susceptible, it is estimated as many
as 2000 of the 9000 species in the south west may be susceptible to
dieback disease (Wills 1993).

Which plants are affected?

Dieback disease generally affects woody shrubs. Grasses, sedges,
rushes, orchids etc. have rarely been recorded as hosts and have
usually been regarded as resistant to infection. However, the dieback
wate mould is exceedingly invasive and is capable of infecting the roots
of many of the so-called 'resistant' species so that even these
species will act as hosts. For example, the primary symptom of
infection - death and discoloration of tissue in the root - has been
observed in some grass species, but rapid root regeneration by the
plants allowed them to survive. Thus, in the case of dieback
disease, the term 'resistant' is applied to many species which
show no immediately apparent secondary symptoms of the disease
(yellowing of the leaves, canopy dieback, and/or death of the plant),
although they may suffer a decrease in vigour. Only a few species
have so far been found which can completely inhibit fungal growth
from the point of entry, and only these can be regarded as truly
resistant. The water mould invades the conducting elements of the plant
and causes the failure of root conductance, and so plants which are
killed die largely as a result of drought.

A study of plant communities in the Stirling Range National Park
assessed 330 species for susceptibility to P. cinnamomi. 36%
were recorded as having at least some individuals in a population
killed by the water mould, with 28% of these highly sensitive to the
pathogen (more than 80% of plants in a population killed). 47% of
woody perennials surveyed were susceptible. Species' susceptibility
generally affected groups of related plants with the result that some
families had large numbers of susceptible species while others were
apparently unaffected by the pathogen.

LANDSAT image of the Stirling Range National Park, Western
Australia.

The four most important woody plant families in Western Australia
are the Myrtaceae (eucalypts, paperbarks, bottlebrushes etc.
making up 737 described species), the Papilionaceae (pea-flowered
plants with 639 described species), the Proteaceae (e.g. Banksia,
Grevillea, Dryandra, Hakea etc. making up 536 described species),
and the Mimosaceae (wattles, 422 described species).

Notably, 85% of species from the Proteaceae assessed for
susceptibility to P. cinnamomi were at least partially
affected by the water mould. My research while at the Department of Conservation
and Land Management (CALM) suggested that as many as 2000 of the 9000
native plant species in south-west Australia may be susceptible to
Phytophthora cinnamomi (Wills 1993).

Why does the water mould cause so much damage?

Variation in annual rainfall has a significant influence on
populations of Phytophthora cinnamomi due to its affect on
soil moisture. Any rainfall pattern promoting moist soil conditions
over summer will allow rapid growth of the water mould within the host and
thus may cause significant disease expression. However, the most
severe impact of the pathogen may well occur after the combination of
autumn and summer rains. Autumn rains would contribute to the early
recharge of soil moisture after summer and allow an increase in
population size of the water mould before growth becomes limited by winter
temperatures. As temperatures begin to rise in spring, populations of
the water mould that had built up in autumn would continue to expand as
long as moisture is available. Substantial rainfall in summer would
then create ideal conditions for the growth and sporulation of P.
cinnamomi.

Substantial plant deaths in the have been reported every few years
in the south west - early in 1974, 1976, 1978, 1982, 1986 and 1990.
In all cases, deaths were preceded by substantial falls of rain in
the previous year, especially in autumn and summer.

Phytophthora cinnamomi has infested many of those habitats
which provided a suitable food-base and year-round moist conditions
for the water mould. However, areas with seasonally dry soils and
dominated by species susceptible to the pathogen are also being
invaded in years when above average rainfall produces high soil
moisture conditions. Variability of summer rainfall is particularly
important since, every few years, heavy summer rains occur,
generating free soil water in the warmest months. These circumstances
not only provide ideal conditions for the growth of this tropical
organism but also favour the initiation and growth of fine roots
which are an important avenue of invasion for the pathogen.

Armillaria

Another serious pathogenic fungus in Western Australia is
Armillaria luteobubalina. Like the dieback fungus,
this fungus can also attack a wide range of plant species,
and is also active in many parts of the south-west. As yet
no substantial surveys have been undertaken to assess the
distribution or impact of Armillaria luteobubalina.
Research so far shows that many areas are at risk including
wandoo woodlands, karri forest and coastal shrubland
communities.

Fruiting bodies of Armillaria luteobubalina.

Cankers

Aerial canker diseases caused by a group of fungi including Diplodena are an increasing problem in Western Australia.
Several aerially-dispersed, canker-causing fungi have been found in
diseased plants from many plant communities over the past decade
by CALM researchers. The cankers, including the fungi
Botryosphaeria ribis and Diplodina sp., have caused
extensive damage to large stands of vegetation on the south coast of
WA, particularly since February 1991. It appears likely that unusual
climatic conditions along the south coast of WA, with 6 months of
serious to severe rainfall deficiency up until May this year, and a
heat-wave lasting four days and reaching 47deg. C, have contributed
to the rapid growth of the cankers observed in native plant
communities since that time. Initial studies reveal that these fungi
also have a broad host range, with 46% of species assessed from a
range of families were damaged by canker fungi. Again, many
Proteaceae were affected, with 82 % of species damaged, and often
killed by the fungus. While canker fungi are not a major problem in
the Jarrah Forest, they are found throughout the south west and have
the potential to cause very serious damage.

Impact of Plant Disease

Plants which provide the main elements of an ecosystem are called
'keystone species' - that is, their presence in an area is critical
to the survival of many of the plants and animals that live there,
too. In large areas of the south-west, the Proteaceae are the most
abundant plant group, acting as keystone species and so providing the
fundamental elements of many plant communities. While species of
Proteaceae are abundant at healthy sites, they are much less
important at sites which had long been infested by Phytophthora
cinnamomi. Other species which display low levels of
susceptibility to the disease, such as sedge and rush species, are
more abundant at sites with a long history of infestation than at
healthy sites.

If a keystone species disappears, it is inevitable that many of
the species that rely on it will also disappear even though these
species are not directly affected by the disease. Plants which must
grow in the shade of a dense canopy may disappear as the susceptible
plants are killed. Animals may suffer from a loss of refuges since
the more open vegetation may offer less protection from
predators.

A white-cheeked honey-eater rests on a Banksia coccinea
killed by canker disease near Cheyne Beach, east of Albany, Western
Australia.

Notably, many species of Proteaceae (e.g.Banksia,
Grevillea, Dryandra) have large flowers that are
pollinated by nectar-eating birds or mammals. The destruction of
large numbers of proteaceous species by wildflower dieback may cause
animal populations to dwindle as the nectar sources on which these
animals rely are eliminated.

Management

Currently, the most practical management technique for the control
of P. cinnamomi in native plant communities is foliar
application of the fungicide phosphonate. Field trials in various
areas in the south-west on plant communities already infested with
Phytophthora cinnamomi have shown that one application of
phosphonate gives excellent control of the disease over several
years.

While canker fungi are not a major problem in south-west
Australia, they are distributed throughout this region and have the
potential to cause very serious damage. Fire appears to be the most
practical management tool for the regeneration of native plant
communities after infestation by canker.